In outdoor photography, the sun at noon will typically come straight from above, causing a face to self-shadow which results in unflattering dark rings under eyes and the neck. If the sun comes from behind the subject, the subject may appear too dark in comparison to the bright background. A common solution to this problem is to use a flash to brighten shadows in a face, or to bring foreground and background to similar brightness. This kind of flash is called a fill-flash as it fills in dark areas of an image with extra light.
Fill-flash, however, has a few shortcomings. Finding the right amount of flash to use, so that the foreground receives just enough light to match the background, is difficult. Too much light causes the foreground to be overexposed (for example, the face becomes too bright), whereas too little flash renders the technique useless and causes details in the darkened areas to be lost. Furthermore, it is hard to estimate the right flash intensity to use as different skin colors reflect more or less light. Similarly, oily patches of skin or flat surfaces such as the forehead cause hotspots which can emphasize sweaty or oily skin and wrinkles.
Another problem with using fill-flash is that the color of the flash and the color of the ambient light often do not agree. Photographers measure light color in terms of a warm or cool temperature expressed in Kelvin (K). For example, indoor lighting is very warm or red in nature (2800 K) compared to a cool flash (5500 K) or outdoor sun (9000+ K). As a result, the color of a subject lit by flash in certain indoor situations looks unnatural. Professional photographers add colored filters to their flash to offset this effect. However, this requires them to carry around a collection of color filters. Additionally, determining the proper filter and using it is often too difficult for the novice photographer.